1. Field of the Invention
The present invention relates to a triangular wave generation circuit for generating triangular waves at a predetermined frequency.
2. Description of the Related Art
Pulse-width modulation techniques are widely used for power supply units such as switching regulators, or motor driver circuits for driving motors, and the like. For a pulse-width modulation, the error of a voltage to be controlled in relation to a target voltage is amplified by an error amplifier, and the error voltage resulting therefrom is compared by a comparator with a periodic voltage with a triangular waveform having a fixed frequency.
To generate periodic voltages of triangular waveforms used in applications as mentioned above, analog triangular wave generation circuits as described in Reference (1) and Reference (2) in the following Related Art List are used widely. FIG. 3 is a circuit diagram illustrating a partially simplified structure of a triangular wave generation circuit as described in References (1) and (2). A conventional triangular wave generation circuit 50 includes a capacitor CT, constant-current sources 52 and 54 for charging and discharging the capacitor CT respectively, two comparators 56 and 58 for setting the upper limit value VH and the lower limit value VL of output voltage respectively, and a flip-flop 60, and a switch SW.
In a known triangular wave generation circuit 50 as shown in FIG. 3, the capacitor CT is charged by the constant-current source 52, and as the output voltage Vout reaches the upper limit value VH, the switch SW is turned on and the electric charge stored in the capacitor CT is discharged by the constant-current source for discharge 54 to lower the output voltage Vout with a constant rate. As the output voltage Vout reaches the lower limit value VL, the switch SW is turned off and the capacitor CT is charged by the constant-current source for charge 52 to raise the output voltage Vout with a constant rate. In this manner, output from the triangular wave generation circuit 50 is an output voltage Vout of a triangular waveform that repeats rising and lowering between the upper limit value VH and the lower limit value VL.
Related Art List
    (1) Japanese Patent Application Laid-Open No. 2004-72657.    (2) Japanese Patent Application Laid-Open No. 2001-345682.
FIGS. 4A and 4B are voltage waveform diagrams of a triangular wave generation circuit 50 as shown in FIG. 3. FIG. 4A shows the output voltage Vout, whereas FIG. 4B shows the voltage Vx at the connection point between the constant-current source 54 and the switch SW. During the period of charging by the constant-current source 52, that is, during the period when the output voltage Vout rises, the switch SW is off, so that the collector or drain of a transistor constituting the constant-current source 54 opens and the voltage Vx is down at ground potential.
When the output voltage Vout reaches the upper limit value VH, the switch SW turns on and discharging by the constant-current source 54 starts. At this time, the electric charge stored in the capacitor CT flows suddenly toward the constant-current source 54, so that the output voltage Vout drops with a steep slope as shown in FIG. 4A and then lowers with a constant rate toward the lower limit value VL.
As described above, the conventional triangular wave generation circuit 50 has a problem that the waveform of the output voltage Vout gets distorted at the time of a charge state switching to a discharge state. This problem can be solved to a certain degree by using a capacitor CT of a larger capacitance, but this brings on another problem of an enlarged circuit area. Moreover, the use of a larger capacitance, which requires the use of larger constant current for charging and discharging, brings about a problem of larger current consumption.